Innate immunity represents the host's first line of defense and relies on immediate recognition of pathogens. A major component of innate immunity is the complement system. Its activation has multiple effects including activation of mast cells, enhanced clearance of bacteria and targeting of antigen to the lymphoid compartment. An important step in complement activation is covalent attachment of activated C3 to pathogen surface as this provides a ligand for complement receptors CD21 and CD35 which are expressed on follicular dendritic cells and certain leukocytes including B lymphocytes and neutrophils. Recent results from biochemical studies and characterization of mice deficient in complement C3, C4 or the receptors CD21/CD35 has demonstrated an important link between innate immunity and regulation of the humoral immune response. The goal of this project is to examine the link between innate immunity and regulation of the repertoire of natural antibody and to define the mechanism of complement activation of mast cells. A murine model of acute septic peritonitis which is mast cell and classical pathway complement dependent will be used to examine the mechanisms of complement protection. The project includes three specific aims: (1) test the hypothesis that the repertoire of natural antibody is regulated by complement; (2) test the hypothesis that peritoneal mast cells are activated by complement receptors CD21/CD35 in the CLP model of acute septic peritonitis; (3) examine the importance of local synthesis of C3 in host defense against bacterial infection in the peritoneum. Understanding the regulation of the repertoire of natural antibody is important both for clarification of basic principles of the immune system and for development of novel therapeutic approaches in host defense against bacterial sepsis. Furthermore, elucidation of the mechanism of complement activation of mast cells is important for defining this important effector function of natural immunity.